XStream can cause Denial of Service via stack overflow

Impact

The vulnerability may allow a remote attacker to terminate the application with a stack overflow error resulting in a denial of service only by manipulating the processed input stream.

Patches

XStream 1.4.20 handles the stack overflow and raises an InputManipulationException instead.

Workarounds

The attack uses the hash code implementation for collections and maps to force recursive hash calculation causing a stack overflow. Following types of the Java runtime are affected:

• java.util.HashMap
• java.util.HashSet
• java.util.Hashtable
• java.util.LinkedHashMap
• java.util.LinkedHashSet
• Other third party collection implementations that use their element's hash code may also be affected

A simple solution is to catch the StackOverflowError in the client code calling XStream.

If your object graph does not use referenced elements at all, you may simply set the NO_REFERENCE mode:

XStream xstream = new XStream();
xstream.setMode(XStream.NO_REFERENCES);

If your object graph contains neither a Hashtable, HashMap nor a HashSet (or one of the linked variants of it) then you can use the security framework to deny the usage of these types:

XStream xstream = new XStream();
xstream.denyTypes(new Class[]{
 java.util.HashMap.class, java.util.HashSet.class, java.util.Hashtable.class, java.util.LinkedHashMap.class, java.util.LinkedHashSet.class
});

Unfortunately these types are very common. If you only use HashMap or HashSet and your XML refers these only as default map or set, you may additionally change the default implementation of java.util.Map and java.util.Set at unmarshalling time::

xstream.addDefaultImplementation(java.util.TreeMap.class, java.util.Map.class);
xstream.addDefaultImplementation(java.util.TreeSet.class, java.util.Set.class);

However, this implies that your application does not care about the implementation of the map and all elements are comparable.

References

See full information about the nature of the vulnerability and the steps to reproduce it in XStream's documentation for CVE-2022-41966.

For more information

If you have any questions or comments about this advisory:

• Open an issue in XStream
• Contact us at